Classification: Bats are mammals and belong to the order Chiroptera, meaning "hand wing."

Size: Full grown free-tails are about 2-5 inches long, weigh one ounce, an have a 10-inch wingspan.

Color: Drab color, ranging from dark brown to gray.

Ears: Their ears are formed to maximize hearing, as well as contribute to lift during flight.

Wings: Long, narrow wings are geared for high speed and maneuverability. The bats can fly-climb to altitudes of 10,000 feet (higher than any other bat) assisted by tail winds, attaining speeds up to 65 mph in flight.

Lifespan: From birth, bats have a 50 percent chance of survival during the first year of life. They have an average life-span of 7-8 years; some live as long as 15 years.

Lips: Prominent wrinkled lips help direct sounds needed for navigation.

Fur: Short, velvety fur helps reduce drag.

Guano: Bat droppings, called guano, are one of the richest fertilizers. Bat guano was once a big business. Guano was the state's largest mineral export before oil.

What bats like to eat: Depending on the season and size of the colony, the bats eat 10,000 to 30,000 pounds of insects a night. Their favorites include Corn ear-worm moths, Codling moths, and June beetles.

Seeing in the dark: Mexican free-tailed bats can find their food in total darkness. The echolocate or located objects by emitting inaudible high-pitched sounds, 10-20 beeps per second and listening to echoes.

Under the bridge: When reconstruction of the 947-foot Congress Avenue bridge was completed in 1980, the Mexican free-tailed bat found the underside vertical crevices to be the perfect roosting site. There are 15 of these crevices across Congress Avenue and 14,000 linear feet of potential bat habitat.

Where the bats go at night On an average night, scientists believe the bats travel an average of 20 miles radius from their roost, often flying inthe same direction. They tend to fly east in search of farmland, where insects are most plentiful.